PROJECT SUMMARY Chronic obstructive pulmonary disease (COPD) is the third leading cause of death in the US. No medical therapies reduce mortality or slow disease progression, in part because COPD is a heterogeneous syndrome, which hinders the development of targeted therapies. The SPIROMICS study enrolled 2,982 participants into a 3-year longitudinal study of current and former smokers and never-smoking controls, phenotyping them clinically, radiographically and biologically to identify sources of heterogeneity in COPD. This new project will extend follow-up of participants enrolled in SPIROMICS, perform a bronchoscopy substudy and perform an exacerbation substudy to test three specific aims. The overarching premise underlying these aims is that COPD is a consequence of a heterogeneous group of molecular phenotypes that underlie distinct radiographic (anatomic) and clinical (physiologic) phenotypes; and that linking molecular phenotypes to specific radiographic and clinical phenotypes will identify key biological factors associated with disease exacerbation and progression. The first aim is to define the natural history of ?Smokers with symptoms despite preserved spirometry? and characterize the airway mucus abnormalities underlying this condition. Patients with this newly recognized condition do not meet current criteria for COPD but nonetheless have symptoms, exacerbation-like events, activity limitations and airway wall thickening and they may have a precursor condition to bona fide COPD. The second aim is to determine the radiographic precursor lesion(s) for emphysema, and identify the molecular phenotypes underlying progression of chronic bronchitis, radiographic airway disease and emphysema. This aim will leverage several novel radiographic methods for measurement of lung disease and new approaches to phenotyping based on inflammatory pathways and the lung microbiome that we developed in SPIROMICS. The third aim is to advance our understanding of the biology of COPD exacerbations through analysis of predisposing baseline phenotypes, exacerbation triggers and host inflammatory response. This aim will leverage RNA sequencing methods to simultaneously measure respiratory pathogens and the host inflammatory response in an exacerbation substudy. Our ultimate goal is to enable targeted approaches to COPD treatment and disease modification that are based on the heterogeneous biological pathways that underlie COPD progression and exacerbations.